Carnegie stages

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrateembryo.

The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos. In the human being only the first 60 days of development are covered; at that point the term embryo is usually replaced with the term fetus.

Primordial embryo. All the genetic material necessary for a new individual, along with some redundant chromosomes, are present within a single plasmalemma. Penetration of the fertilising sperm allows the oocyte to resume meiosis and the polar body is extruded.

Pronuclear embryo. Two separate haploid components are present - the maternal and paternal pronuclei. The pronunclei move towards each other and eventually compress their envelopes where they lie adjacent near the centre of the wall.

TEM inspection of in vitro blastocysts has allowed us to identify two types of cells that the developing embryo apparently discards. These are sequestered cells and isolated cells. Sequestered cels are groups of cells that are located in between the zona pellucida and the trophoblast. Isolated cells are mainly found in the blastocystic cavity.